1. Field of the Invention
The present invention relates to fuel cells and components therefore. In particular this invention relates to membranes, membrane electrode assemblies and fuel cells employing the membranes and membrane electrode assemblies.
2. Description of the Related Art
A variety of electrochemical cells fall within a category of cells referred to as ion exchange membrane (IEM) cells. An IEM cell employs a membrane comprising an ion-exchange polymer. This ion-exchange polymer membrane serves as a physical separator between the anode and cathode while also serving as an electrolyte. IEM cells can be operated as electrolytic cells for the production of electrochemical products, or as fuel cells for the production of electrical energy. The best-known fuel cells are those using a gaseous fuel such as hydrogen, with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol.
Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. In contrast to batteries, which must be recharged, electrical energy from fuel cells can be produced for as long as the fuels, e.g., methanol or hydrogen, are supplied. Thus, a considerable interest exists in the design of improved fuel cells to fill future energy needs.
In many fuel cells, a cation exchange membrane is used wherein protons are transported across the membrane as the cell is operated. Such cells are often referred to as proton exchange membrane (PEM) cells. For example, in a cell employing the hydrogen/oxygen couple, hydrogen molecules (fuel) at the anode are oxidized by donating electrons to the anode, while at the cathode the oxygen (oxidant) is reduced by accepting electrons from the cathode. The H+ ions (protons) formed at the anode migrate through the membrane to the cathode and combine with the reduced oxygen to form water. In many fuel cells, the anode and/or cathode comprises a layer of electrically conductive, catalytically active particles, usually in a polymeric binder such as polytetrafluoroethylene (PTFE), on the proton exchange membrane. Alternatively, the anode and the cathode layers are applied to the gas diffusion structure. The gas diffusion structure allows entry of the fuel or oxidant to the cell. The gas diffusion/electrode structure is hot pressed to the membrane. The resulting structure consisting of the membrane, electrodes and optional gas diffusion structure is referred to as a membrane electrode assembly (MEA). The manner of fabricating a high performing MEA depends strongly on the properties of the membrane. Factors such as the membrane chemical solubility, thermal stability, and mechanical strength are important.
One drawback of presently known fuel cells is the so-called crossover of fuel through the membrane. The term crossover refers to the undesirable transport of fuel through the membrane from the fuel electrode, or anode, side to the oxygen electrode, or cathode side of the fuel cell. After having been transported across the membrane, the fuel will either evaporate into the circulating oxygen stream or react with the oxygen at the oxygen electrode. Fuel crossover diminishes cell performance for two primary reasons. Firstly, the transported fuel cannot react electrochemically to produce useful energy, and therefore contributes directly to a loss of fuel efficiency (effectively a fuel leak). Secondly, the transported fuel interacts with the cathode, i.e., the oxygen electrode, and lowers its operating potential and hence the overall cell voltage. The reduction of cell voltage lowers specific cell power output and reduces the overall efficiency.
One particularly useful group of cation-exchange materials for membranes in PEM cells is perfluorinated sulfonic acid polymers such as NAFION(copyright), available from E.I. duPont de Nemours and Co. Such cation-exchange polymers, when cast into films for membranes, have good conductivity and chemical and thermal stability, which provide long service life before replacement.
Perfluorinated sulfonic acid polymers such as NAFION(copyright) and other ion exchange materials have also been incorporated into films, for example porous polytetrafluoroethylene (PTFE), to form composite membranes, as described for example in U.S. Pat. No. 5,082,472, to Mallouk, et al.; JP Laid-Open Nos. 62-240627, 62280230, and 62-280231; U.S. Pat. No. 5,094,895 to Branca; U.S. Pat. No. 5,183,545 to Branca et al.; and U.S. Pat. No. 5,547,551 to Bahar, et al. Each of the foregoing references is incorporated herein in their entirety.
Another approach to construction of an ion exchange membrane is described in PCT/US96/03804 to Grot, et al. Grot et al. disclose a composite membrane with a thickness of less than 250 xcexcm prepared by precipitation of a water-insoluble, inorganic conductor such as zirconium hydrogen phosphate into a porous NAFION(copyright) membrane. However, the composite membrane exhibits a very high amount of crossover, especially when operated at high temperatures. Similarly, S. Malhotra, et al., in Journal of the Electrochemical Society, Vol. 144, No. 2, L23-L26, 1997 disclose a NAFION(copyright) membrane containing phosphotungstic acid. Although the resulting composite membrane was said to demonstrate high conductivity at elevated temperature, the membrane lacked sufficient strength at reduced thickness for hydrogen fuel cell applications.
One significant drawback to the use of NAFION(copyright) and related polymers in membranes, composite membranes, and catalyst layers is that such materials are not efficient at high temperatures, especially the high temperatures seen in systems incorporating steam reforming or partial oxidation of fuel. The difficulty of on-board storage and refueling of hydrogen is a major concern in the application of hydrogen fuel cells in vehicles. One approach for surmounting this obstacle has been to utilize the hydrogen fuel obtained through steam reforming or partial oxidation of gasoline. Since hydrogen fuel from this source usually contains a trace amount of carbon monoxide, which causes severe poisoning of anode catalysts, such fuel cells are operated at high temperature to prevent carbon monoxide adsorption onto the anode catalysts. At these elevated temperatures, membranes comprising perfluorinated sulfonic acid polymers such a NAFION(copyright) quickly lose ionic conductivity due to dehydration.
There accordingly remains a need for an MEA with a membrane that maintains functionality in hydrogen fuel cells wherein hydrogen fuel contains trace carbon monoxide from the fuel processing. There further remains a need for a membrane exhibiting sufficient strength at reduced thickness, maintaining high conductance at elevated temperature, and minimum fuel crossover for hydrogen fuel cell applications.
The above described drawbacks and disadvantages are overcome or alleviated by a composite membrane structure comprising a composite membrane and at least one protective layer disposed adjacent to the composite membrane. The composite membrane comprises a porous polymeric matrix and an ionically conductive solid, noble metal or combination thereof dispersed within the matrix, and preferably, a binder. The binder is preferably an ion exchange polymer. The protective layer comprises binder and ionically conductive solid, hygroscopic fine powder or a combination thereof.
In one embodiment the composite membrane comprises a porous polymeric matrix and an ionically conductive solid. The ionically conductive solid is dispersed within the matrix. The composite membrane preferably also comprises a binder.
In another embodiment the composite membrane comprises a porous polymeric membrane, a binder, and a noble metal. The noble metal is dispersed within the matrix.
In another embodiment the composite membrane comprises a porous polymeric membrane, a binder, ionically conductive solid, and a noble metal. The noble metal is dispersed within the matrix.
The composite membrane of any of the embodiments, in combination with at least one protective layer, forms a composite membrane structure. The protective layer comprises binder and ionically conductive solid, hygroscopic fine powder or a combination thereof.
Alternatively, in a further embodiment the composite membrane comprises an ionically conductive solid, a binder and support polymer. The membrane is formed by casting a solution of the support polymer, ionically conductive solid and binder to form a film. The film may optionally be combined with a protective layer as described above.
The cast composite membrane or any of the above mentioned composite membrane structures, may be employed in a membrane electrode assembly (MEA) comprising a composite membrane structure or cast composite membrane, an anode, a cathode, and optional current collectors.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.